(1) Field of the Invention
This invention applies to voltage regulators, and more particularly to a 3-phase alternating current (AC) electronic tap-changing voltage regulator. The present invention provides a specific transformer winding topology and commutation technique that improves performance and reduces cost compared to conventional methods.
(2) Background Art
Tap changing transformers are commonly used to regulate AC voltage in both low power, low voltage applications, and high power applications at distribution level voltages. Distribution level regulators typically consist of a multi-tapped transformer winding coupled to a mechanical tap changer so that regulation within +/−10% of nominal voltage is possible. These tap changer designs incorporate various mechanisms to ensure that, when transitioning from one tap to the next under load conditions, load current is not interrupted and arcing and inter-tap short circuit current are minimized.
In low voltage (e.g., less than about 1000V) and lower power applications (e.g., less than about 1 MVA) mechanical tap changers are often implemented using a simpler design incorporating a sliding commutation brush which can be positioned at arbitrary points along an exposed transformer winding in order to achieve the change in effective turns ratio. This technique has much lower cost than a discrete tap changer of the type used at higher power levels, but does not provide the same performance and also requires more maintenance.
Electronic tap changers are also commonly used in low voltage and low (e.g., less than about 1 kVA) to moderate (e.g., about 500 kVA) power levels. Referring now to FIGS. 1-3, three known devices are shown. In FIG. 1, an electronic tap changer 10 comprises an electronic switch 20, 22, 24 connected to each tap 12, 14, 16 of a multi-tapped transformer 40 or auto transformer. Typically, each switch 20, 22, 24 includes back-to-back connected silicon controlled rectifiers (SCRs) 30, due to their low cost, simplicity, and ruggedness. By actively selecting which SCRs 30 are firing (e.g., by using appropriate sensing and gating controls, for example), the effective turns ratio of the transformer 40 can be controlled, so that the output voltage may be varied for a constant input voltage (as supplied by an AC voltage source 50), or, in the case of a regulator, the output voltage may be made constant within a certain tolerance under conditions of varying input voltage. Tap changer 10 may include other components, as would be recognized by one of ordinary skill in the art, including, for example, ground connections 32, loads 34, etc.
An alternative implementation to the basic electronic tap changer 10 (FIG. 1) is shown in FIG. 2. Here, a series transformer secondary winding 60 reduces the current through the electronic switches 20, 22, 24, while increasing the voltage withstand capability of each switch.
In any SCR-based on load tap changer, provisions must be made to avoid both load current discontinuity and high inter-tap circulating current when commutating the load current from one set of active SCRs to another (i.e., making a tap change). This is the same fundamental problem which must be addressed in the design of high power, discrete mechanical-on-load tap changers. The unique problem in the case of SCR based tap changers is a result of the gating characteristics of SCRs. That is, SCRs may be turned on at any arbitrary time, but will only cease to conduct when the load current naturally falls to zero (normally once each electrical half cycle).
When commutating from an ‘old’ tap to a ‘new’ tap, if the new tap SCR is fired before the old tap SCR has ceased conducting, short circuit current will potentially flow between the two taps until the old tap SCR current flows through current zero. This current overload is potentially damaging to the SCRs and transformer windings, and may result in a voltage drop as the short circuit current flows through the source impedance. Conversely, if a delay is used such that the old tap SCR is allowed sufficient time to turn off and regain its voltage blocking ability before the new tap SCR is activated, the current discontinuity which exists during the delay period may result in damaging or unacceptable voltage transients for inductive loads.
Referring now to FIG. 3, this problem can be solved by adding a commutating current path 70 through an impedance element (e.g., commutation resistor 80). This is a basic representation of one of many methods commonly utilized in high power, mechanical tap changers. In a device according to FIG. 3, when commutating from tap 12 to tap 14, for example, the SCR pair 26 connected to the commutation resistor 80 is first gated, resulting in short circuit current between the two taps 12, 14, which is limited by resistor 80 to an acceptable level. After the tap 12 conducting SCR 20 has naturally ceased to conduct, the tap 14 SCRs 22, 26 may be fired after some delay but with no concern for a current discontinuity as the load current may continue to flow through the resistor 80 until the tap 14 SCRs 22, 26 are conducting, at which time the gate signals of SCR pair 26 are removed.
The wiring scheme of FIG. 3, or one of its known derivatives, could be implemented on each tap in a 3-phase regulator in order to implement an acceptable commutation scheme for all possible tap changes. The additional complexity of this scheme, however, results in a substantial additional cost which may render the entire device impractical, and the additional control complexity and parts count reduces the reliability of the device.